Technical Field
The present invention relates to a method for controlling a generator of electric energy which is connected to an electric supply grid in a grid connection point. In addition, the present invention relates to such a generator of electric energy.
Description of the Related Art
Feeding electric energy into an electric supply grid, such as the European grid or the US power grid, is generally known. The electric supply grid as described below refers to the widely accepted AC voltage grid. This does not exclude the presence of DC voltage sections in the grid. Likewise, frequency-independent aspects may generally also refer to a DC voltage grid. Historically, energy is fed into an electric supply grid with a large power station that drives a synchronous generator using primary energy, such as coal, nuclear energy or gas. Depending on the number of pole pairs and the speed of the synchronous generator, this generator feeds into the supply grid with a certain frequency. The synchronous generator may be technically controlled so as, for example, to adjust the output. However, such adjustment process can take a long time.
With changing situations in the supply grid that is to be fed, the physical reaction of the synchronous generator often causes a change in the grid condition, at least for a short time. For example, the speed of the synchronous generator increases if the supply grid is not able to take the power completely that is or can be provided by the synchronous generator. This excess power then accelerates the synchronous generator, which results in an increased feeding frequency. Accordingly, the frequency in the supply grid may increase.
When feeding into a supply grid, the grid stability must also be taken into account. The loss of grid stability, i.e., the loss of the stability of the supply grid, may result in the feeding generator being powered off. Such a loss of stability, which is referred to as such and abbreviated as “LOS” among German-speaking experts, describes physical processes that no longer allow for a continued operation and must be terminated by cutoffs. In the case of power plants, this affects their output, and can thus contribute to an escalation of the so-called deficit output. In the worst case, this loss of stability leads to a total energy system failure due to error cascading and deficit accumulation. Such total failures are very rare; however, one occurred in Italy on 24 Sep. 2004.
Loss of grid stability, i.e., the so-called loss of stability, is a phenomenon which involves at first a loss of angular stability that may eventually cause a loss of voltage stability.
Overcurrents to be achieved are determined as stability criteria, which can be provided in the case of a loss of stability. This requires the systems to have a certain design. A new power plant, in particular a power plant that is to be newly built, is thus coordinated to the supply grid as is represented at the grid connection point to which the power plant is to be connected.
When connecting large power plants to an electric supply grid, the short circuit current ratio is an important criterion; this is known among German specialists as “short circuit radio” and abbreviated as “Scr”. This short circuit current ratio is the ratio of the short circuit power to the connected load. Short circuit power is the power that the respective supply grid on the considered grid connection point, to which the power plant is to be connected, can provide in the case of a short circuit. The connected load is the connected load of the power plant that is to be connected, in particular the nominal capacity of the generator that is to be connected.
To secure reliable operation, i.e., to avoid a loss of stability to the greatest extent possible, power plants are generally designed for the respective grid connection point in such a way that the short circuit current ratio is higher than 10, normally even higher than 15. The supply grid can then provide a relatively high short circuit power at the grid connection point. That means that the grid has a low grid impedance and is referred to as a strong grid.
In the case of a weak grid, in other words, in the presence of a high impedance, feeding is only possible with a low connected load, i.e., only a power plant with a low connected load can be connected. This usually leads to the fact that either a new power plant cannot be connected to such a grid connection point, or the grid has to be changed, particularly by equipping it with further, more powerful lines. This is generally referred to as grid reinforcement.
For feeding electric energy by decentralized generation units, in particular wind power installations, the problem of the grid's loss of stability is basically unknown. Already at the end of the nineties, first proposals were made to ensure that wind power installations also contribute to the electric support of the grid. This, however, does not take into account the cause of a loss of stability, in particular that feeding into the supply grid can cause a loss of stability.
For example, the German patent application U.S. Pat. No. 6,891,281 describes a method in which wind power installations can change and, in particular, reduce their power feed-in depending on the grid frequency. U.S. Pat. No. 7,462,946 suggests that in the case of a grid failure, particular in the case of a short circuit, a wind power installation limits the power that it feeds-in instead of being disconnected from the grid in order to achieve a grid support. U.S. Pat. No. 6,965,174 describes a method for supporting the grid by means of a wind power installation that, depending on the grid voltage, adjusts a phase angle of the fed-in electricity, and thus feeds reactive power into the grid depending on the voltage so as to support the grid. U.S. Pat. No. 6,984,898 also relates to a method for supporting the grid by means of a wind power installation in which the wind power installation reduces, depending on the grid voltage, the power that is to be fed into the grid, particularly so as to avoid a disconnection from the grid in order to support the grid by means of a wind power installation.
The fact that such decentralized generation units, such as wind power installations, may be the underlying cause for the loss of stability in the grid has not been taken into account. In the essay “Loss of (Angle) Stability of Wind Power Installations” by V. Diedrichs et al., submitted for and presented at the “10th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Grids for Offshore Wind Farms, Aarhus (Denmark), 25-26 Oct. 2011”. There, reference was basically made to the problem that the loss of stability in the grid can basically also occur for wind power installations that are connected to the supply grid for feed-in. This essay essentially raises awareness of the problem and is hereby incorporated herein by reference in its entirety. In particular, its technical explanations also apply to the present application.
Basically, findings, experience and other knowledge of the operation and connection of large power plants to the electric supply grid cannot be transferred to wind power installations, including large wind parks with numerous wind power installations that are connected to the supply grid for feed-in. The responsible expert who connects a power plant to a supply grid in order to operate it there is already a different expert from the one wanting to connect a wind power installation to the supply grid in order to operate it there. Wind power installations—and much of the following also applies to decentralized generation units—depend on wind and must therefore take a fluctuating energy source into account; they usually do not feed into the supply grid with a synchronous generator that is directly coupled to the grid, but use a voltage-based inverter instead; they have a different size than large power plants, whereby their nominal capacity is usually 3 powers of ten below that of a large power plant; they are usually subject to other political laws which often ensure the acceptance of the provision of power by the operators of electric supply grids; they are usually decentralized; and, they usually feed into a medium-voltage grid, whereas large power plants usually feed into an extra high voltage grid.
The German Patent and Trademark Office has found the following prior art in the priority application for the present application: DE 10 2011 053 237 A1, WO 2010/060903 A1, US 2010/0148508 A1, DE 10 2007 044 601 A1, DE 10 2007 018 888 A1, US 2010/0237834 A1 as well as the article by Volker Diedrichs et al., “Loss of (Angle) Stability of Wind Power Plants—The Underestimated Phenomenon in Case of Very Low Short Circuit Ratio-”.